This invention relates to a laminated glazing pane composed of two or more sheets of vitreous material, with successive facing surfaces of the sheets being bonded together via an intervening layer of adhesive material. The invention is particularly directed to the acoustic properties of such panes.
The use of large areas of windows and other panels such as glazed partitions which is a feature of modern architectural practice, gives rise to the problem of achieving comfortable sound levels in rooms whose walls are constituted by the panels, especially in noisy environments. The problem is particularly acute in the case of windows which face busy roads or which are near airports, and glazing panels having good sound insulating properties are required for use in these situations, and also to form interior partitions such as in recording and broadcasting studios.
A pane formed by a single sheet of glass, such as an ordinary plate glass window, gives rise to a transmission loss, i.e. sound reduction, whose extent depends on the dimensions of the sheet and the frequency of the incident sound. For a given sheet, the transmission loss will be greater for higher sound frequencies within a certain sound frequency range; for a given frequency of sound within that range incident on a sheet of given length and breadth, the transmission loss will increase with increase in the thickness of the sheet.
Thus, in order to increase the sound insulating effectiveness of a sheet, it would seem that it is only necessary to increase the thickness of the sheet.
Increasing the thickness of the sheet however also has an effect on the extent of the frequency range referred to.
This range is bounded at its lower end by the frequency of sound vibration which corresponds with the fundamental resonance frequency of the sheet. A substantial decrease in transmission loss takes place at or about the resonance frequency.
It has been calculated that the fundamental resonance frequency of a sheet of given length and breadth is directly proportional to the thickness of the sheet.
Above the resonance frequency zone, transmission loss at a given sheet increases with the frequency of the incident sound as above described, until a frequency is reached which gives rise to the so-called `co-incidence effect`, The frequency of sound waves giving rise to the co-incidence effect depends, for a given sheet, upon the angle of incidence of such waves on the sheet, and corresponds to the frequency at which the projected incident wavelength on the sheet is equal to the wavelength of free bending waves in the sheet. This wavelength is the one of the elastic wave which freely propagates along the plane of the sheet assumed of infinite dimension; it depends upon the nature of the material and the thickness of the sheet. The lowest sound frequency at which co-incidence takes place, the critical frequency of co-incidence, is that corresponding to a sound wavelength equal to the wavelength of free bending waves in the sheet, when the angle of incidence of the sound is 90.degree. , i.e., grazing incidence, to known an incidence tangent to the surface of the sheet. Where the co-incidence effect obtains, the transmission loss across the sheet is reduced because of the efficient mechanical coupling existing between the sheet and the surrounding air.
This critical frequency is independent of the length and breadth of the sheet, but decreases as the thickness of the sheet is increased.
Thus it will be seen that in order to increase the sound transmission loss across a sheet, the sheet may be given an increased thickness, but such an increase in the thickness of the sheet will narrow the range of sound frequencies over which the increased sound transmission loss is obtained.
For example, considering single sheets of glass one meter square, two sheets, 6 mm and 12 mm thick, respectively, may have resonance frequencies of approximately 30 ;l Hz and 60 Hz, respectively, and critical frequencies of co-incidence of approximately 2000 Hz and 1000 Hz, respectively.
Currently accepted theories predict that doubling the thickness of a sheet in this way gives a 6 dB increase in sound transmission loss for sound of a given frequency, but in the cases being considered, this increase in transmission loss will only be obtained over the range from 60 Hz to 1000 Hz, because in the ranges 30 Hz to 60 Hz and 1000 Hz to 2000 Hz, the advantage of doubling the sheet thickness is masked by resonance or co-incidence effects. It should be noted that the numerical values given here are based on theory and are only approximately borne out by experiment.
In order to enhance the sound insulating properties of a pane, it is known to increase the transmission loss of the pane by substituting for a single sheet, two or more thinner vitreous sheets of the same total thickness which are bonded together by means of a layer or layers of intervening adhesive material.
The transmission loss at the critical frequency for such pane will be somewhat greater than the transmission loss at the critical frequency for the thick single sheet and this will ensure improved acoustic insulation; however this improvement is usually not sufficient.
One way of increasing the transmission loss has been to increase the thickness of one such intervening layer, although this adds appreciably to the cost of the pane.
A factor which contributes to the sound insulating effectiveness of such a panel is the damping afforded by the intervening layer or layers of adhesive material. To measure this damping, applicants have devised a test for comparing the behavior of a laminated pane and a monolithic pane constituted by a single vitreous sheet when these are subjected to vibration.
The test is carried out at 20.degree. C. In order to perform the test, the thickness of each individual vitreous sheet of the laminated pane is measured, and a monolithic vitreous pane having a thickness equal to the total measured thickness of the vitreous sheets of the laminated pane is cut to form a monolithic bar 20cm long and 2 cm wide. A laminated bar of similar size is also cut from the laminated pane, the laminations succeeding one another in the thickness direction. The monolithic bar is supported at points lying 5 cm from each of its ends, and one end is caused to vibrate at a variable frequency with substantially constant amplitude. The amplitudes of the vibrations transmitted to the other end of the monolithic bar are monitored and plotted on a graph against the frequency of the vibrations.
It is found that such graph shows a series of amplitude peaks of different heights corresponding to various frequency zones. The highest amplitude peak occurs at what applicants call the base frequency (f*) of the monolithic bar. This base frequency can be converted to a theoretical base frequency (f.sub.0) which is the frequency at which the highest amplitude peak would occur for the laminated bar if it were vibrated in the same way, provided that no damping took place in the adhesive interlayer or interlayers of the laminate. This theoretical base frequency can be expressed by ##EQU1## where
e.sub.v = the total thickness of the vitreous sheets of the laminated bar, and is equal to the thickness of the monolithic bar,
P.sub.v = the density of the vitreous material,
e.sub.i = the total thickness of the adhesive intervening layer(s) of the laminated bar, and
P.sub.i = the density of such adhesive material.
After the test on the monolithic bar, the laminated bar is likewise supported at points lying 5 cm from each of its ends and one end is caused to vibrate at a variable frequency with substantially constant amplitude. The amplitudes of the vibrations transmitted to the other end of the laminated bar are monitored and plotted on a graph against the frequency of the vibrations. The highest amplitude peak on this graph occurs at what applicants call the measured base frequency (f) of the laminated bar. This measured base frequency (f) of the laminated bar can be compared to the theoretical base frequecy (f.sub.0) obtained from the test on the monolithic bar described above.